1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Discussion of the Background
Conventionally, an image forming apparatus of electrophotography has included an electrifier to electrify the surface of each photoconductor. As this electrifier, there have been known contact electrifiers of, for example, a roller type and a blade type. Moreover, among such contact electrifiers, there have been known electrifiers to which an electrifying voltage having an alternating-current (AC) voltage superposed on a direct-current (DC) voltage is applied. It should be noted that in the following description, not only an electrifier in direct contact with a photoconductor but also an electrifier not in contact but closely adjacent will be referred to as a contact electrifier.
When the AC voltage is applied, a contact electrifier causes discharge between the electrifier and a photoconductor to appropriately electrify the surface of the photoconductor. Excessive discharge caused by the electrifier may damage the photoconductor. In view of this, the magnitude of AC component of the electrifying voltage applied to the electrifier is controlled to maintain an amount of discharge within a suitable range (see Japanese Unexamined Patent Application Publication No. 2001-201920 and Japanese Unexamined Patent Application Publication No. 2007-199094). Furthermore, image forming apparatuses recited in Japanese Unexamined Patent Application Publication No. 2001-201920 and Japanese Unexamined Patent Application Publication No. 2007-199094 include environment sensors to detect environmental changes inside of the apparatuses such as temperature and humidity. In accordance with the environmental changes inside of the apparatuses detected by such environment sensors, AC component of the electrifying voltage applied to the electrifier is controlled.
The contents of Japanese Unexamined Patent Application Publication No. 2001-201920 and Japanese Unexamined Patent Application Publication No. 2007-199094 are incorporated herein by reference in their entirety.
Recently, there has been a demand for increasing the thickness of a photosensitive layer to prolong the service life of a photoconductor. Therefore, as the frequency of use of the photoconductor increases, the photosensitive layer becomes thinner than an initial state. Consequently, application of the electrifying voltage having AC component set in the initial state may unfortunately cause excessive discharge with respect to the photoconductor.
In this respect, in the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-201920, the AC component of the electrifying voltage is set based on a plurality of measurement points in the initial stage. However, the AC component of the electrifying voltage is then set based on a value measured in the printing step and a setting log. This decreases setting accuracy. Also, in the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-199094, there is only one measurement point to cause discharge with respect to the photoconductor. Similarly to the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-201920, setting accuracy of the AC component of the electrifying voltage is not high. Therefore, when the image forming apparatuses disclosed in Japanese Unexamined Patent Application Publication No. 2001-201920 and Japanese Unexamined Patent Application Publication No. 2007-199094 include the photoconductor having a thick photosensitive layer in the initial state, it is difficult to set the optimum electrifying voltage depending on states of use.
In view of the above-described problems, it is an object of the present invention to provide an image forming apparatus to set the optimum electrifying voltage even though a photoconductor having a thick photosensitive layer is used.